Both linear and rotary filling machines are known to comprise a plurality of serially arranged operating units for the bottles to be moved therethrough. Plastic containers like PET bottles, particularly, are moved by being hooked at the neck by means of suitable grippers that are usually made of steel or resistant plastic. These grippers are controlled by suitable cams, which provide to close them around a bottle neck in the gripping stations, and then hold the same in this condition throughout the pathway to the release station.
However, in a filling machine, several bottle sizes can be usually processed. For each bottle size a different neck can be provided, so that the same gripper is not suitable for all the bottle types. In particular, the different diameters of the bottle necks in the various types needs that the jaws of the grippers be adapted for each diameter. This requires that the distribution star on which the grippers are mounted be disassembled and substituted with another star wherein the grippers suitable for the new diameter are mounted. This operation is lengthy and difficult, so that a relatively long stop of the plant is needed.